1. Field of the Invention
At least one embodiment of the invention relates to a device for medical applications, which comprises an elongated, conductive element having a distal end. Such an elongated, conductive element can be an electrode lead, for example.
2. Description of the Related Art
Such devices are used e.g. in an implantable defibrillation arrangement 100 of the type known e.g. from U.S. Pat. No. 5,531,766 and illustrated in FIG. 1. In order to stimulate the heart H of the patient P, a defibrillator 110 is electrically connected to a lead 101 which, in this case, comprises a special electrode in the form of a shock electrode 104 disposed on the distal end thereof and which is placed in the patient's heart. Defibrillation typically takes place in a monopolar manner via a current path between electrode 104 and an electrode (not shown), which is disposed more or less further away from the heart H to be stimulated. For example, the housing of defibrillator 110, which contains the units for the detection of cardiac signals and for generating electrical impulses, can function as counter-electrode.
FIG. 2 shows the distal end of another lead 201. The tip of the lead comprises two electrodes 204 and 208 for performing a so-called bipolar cardioversion. While electrode 204 forms a contact at the tip of lead 201, electrode 208 is designed as an annular contact on the circumference of the lead. The supply lead comprises two spiral-wound leads 202 and 207, which electrically connect electrodes 204 and 208 to a connecting plug (not shown) on the proximal end of the lead.
At this time, patients who have a conventional defibrillation arrangement according to FIG. 1 cannot be examined using a magnetic resonance tomograph (MRT) or an MRI scanner, since the strong electromagnetic alternating fields can induce intense heating of the lead tip and thereby damage the surrounding tissue.
FIG. 3 shows a typical course of the temperature of a lead tip of a defibrillation arrangement in a magnetic resonance tomograph. When the electromagnetic field is switched on (reference numeral 301), the temperature of the lead tip increases abruptly, wherein the steepness of the increase and the maximum temperature are strongly dependent on the position of the lead relative to the electromagnetic field of the magnetic resonance tomograph. When the electromagnetic alternating field is switched off (reference numeral 302), the lead tip cools down relatively quickly due to the relatively low heat capacity thereof.
To solve the above-described heating problem, U.S. patent application publication 2009/0105789 A1 proposes use of a temperature-dependent switch, which disconnects the lead tip from the supply lead if heating occurs. However, such a switch has a distinct hysteresis, the temperature range is difficult to set, and the lead tip can only be switched fully on or off, and therefore, in the switched-off state, it is not possible to stimulate the heart or derive signals.
In addition, U.S. patent application publication 2010/0174348 A1 discloses an implantable electrical lead, the use of which is compatible with an MRI scanner. The lead comprises two parallel resonators which are formed by concentrically coiled insulated lead wires in the interior thereof, and which are tuned to a Larmor frequency of the tissue of a living organism in which the lead will be used. As an additional mechanism for suppressing unwanted influences of the strong electromagnetic field with HF components which exists in an MRI scanner, blocking elements (cable traps) are provided along the lead to prevent currents from being induced in the lead by the HF components. Blocking elements are designed, for example, as interspaced metallic hollow cylinders on the outer circumference of the lead.